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    Lunar distance by camera, 1893
    From: Paul Hirose
    Date: 2011 Nov 12, 21:50 -0800

    The 1893 annual report of the U.S. Coast and Geodetic Survey has an
    appendix on a test of a photographic method for determining time and 
    longitude with an ordinary portable camera. It's a translation of an 
    article by C. Runge of Hanover, Prussia. He thought the method might be 
    useful to exploring parties. Sextant lunar distance observations would 
    be replaced by a simple photographic procedure. The images could be 
    reduced later at headquarters by a specialist.
    
    In Runge's experiment, a single plate received multiple exposures, which 
    included 1) several instantaneous shots of the Moon, and 2) a time 
    exposure of stars trailing through the area of the plate formerly 
    occupied by the Moon. The camera remained stationary for the whole sequence.
    
    Watch time was recorded for each Moon exposure. During the star trail 
    exposure, Runge made a break in the trails every 5 or 10 minutes by 
    obstructing the lens for a few seconds. The watch time of each break was 
    recorded.
    
    There are three ways to determine time from the photo.
    
    1. By right ascension. A line drawn with a needle and straightedge on
    the emulsion, perpendicular to the star trails, represents a meridian of
    LHA. The watch time when a star had that LHA may be ascertained from the
    positions of the breaks in its trail with respect to the line. Likewise,
    the corresponding time for a Moon image may be calculated from its
    distance from the line, the more accurately if the line is very close to 
    the limb. From the time difference, the Moon's right ascension is 
    obtained, then converted to UT by interpolating the almanac.
    
    2. By declination. Each star trail is a circle of known declination, so 
    the Moon's declination at the time of an exposure may be deduced. 
    Naturally, this method requires that the Moon have a considerable rate 
    in declination. During the experiment it was about 15" per minute of 
    time. The author estimated a single measurement would be good to about 
    20", hence roughly a minute of time, but remember an average may be 
    taken with the multiple Moon exposures.
    
    3. By lunar distance. A break in a star trail is effectively a star 
    whose right ascension is the actual RA of the star, plus a correction 
    for the elapsed time between the Moon exposure and the star trail break. 
    The separation angle from the Moon to the "star" may be measured and 
    reduced to UT in the usual way.
    
    In each of these methods the measured angle is topocentric, and has to
    be corrected to geocentric. That requires latitude and local time. In my 
    view it rather defeats the objective, since the exploring party must 
    still make latitude and time sights.
    
    The author tested these ideas with a single plate taken on the night of
    1893 June 17 in Hanover with a 240 mm f/14 lens. He carefully set the
    camera on a window sill and took 8 shots of the new Moon between 2200 
    and 2223 local. Then from 2250 to 0040 he allowed the stars of Leo to 
    trail through that part of the frame. With the three methods outlined 
    above he got, respectively, 39.1, 39.1, and 38.6 minutes (mean = 38.9) 
    for the difference between local time and Greenwich. The correct value 
    was 38.9.
    
    Runge wrote, "This close result seems to partake of the nature of a 
    coincidence. Still, we believe that an error of more than .2 m is 
    precluded if the measurements on the plate are made as carefully as the 
    one just described." He used a measuring machine (normally employed to 
    read spectrograms) with a precision screw to position the plate under a 
    microscope.
    
    Advantages of the Runge method:
    
    1. No need for compromise exposure settings. The Moon shots can be 
    optimized for a sharp limb, and the star shot made with wide open 
    aperture and as long an exposure as desired.
    
    2. By choosing the time of the star exposure, the star field can be 
    shifted east or west to bring it near the Moon in the image. This lets 
    you use a longer lens for a more favorable image scale. It also reduces 
    the effect of refraction, since both Moon and stars have about the same 
    altitude when photographed.
    
    Disadvantages:
    
    1. Reduction is even more complex than a sextant lunar.
    
    2. Camera must not move during the whole sequence of exposures.
    
    3. The long time exposure of the stars requires frequent attention by 
    the observer.
    
    4. You still need a sextant for the latitude and time sights.
    
    5. Unusable on the water.
    
    The modern hobbyist would find 2 and 3 especially irksome. Perhaps a 
    reasonable compromise would be to get a few Moon shots, then proceed 
    without much delay to the star shot (or vice versa). The star shot would 
    be just long enough to form noticeable trails. (When you reach the point 
    where trails begin to form, a longer exposure won't show dimmer stars, 
    just longer trails.) Of course the star shot will also capture an 
    overexposed Moon, but with planning it'll be clear of the good Moon images.
    
    With a digital camera the measuring would be easy. Even a simple program 
    like Windows Paint lets you magnify a JPEG image and read any point's xy 
    coordinates in pixels.
    
    Nevertheless, the lunar enthusiast of today will probably prefer to 
    shoot lunars with a sextant. I mention this photographic method as a 
    curiousity.
    
    "On Photography as Applied to Obtain an Instantaneous Record of Lunar
    Distances for Determinations of Longitude", U.S. Coast and Geodetic
    Survey Annual Report (1893), Appendix No. 4.
    
    http://docs.lib.noaa.gov/rescue/cgs/data_rescue_cgs_annual_reports.html
    
    Select document "1893 pt 2". The document size is 35 MB.
    
    It's a translation from the original in German, published in 1893 in 
    Zeitschrift für Vermessungswesen, available at archive.org:
    http://www.archive.org/details/zeitschriftfrve23vermgoog
    
    Specifically, the following URL should open the Runge article in the 
    online reader (I have never seen the reader to work with Internet Explorer):
    http://www.archive.org/stream/zeitschriftfrve23vermgoog#page/n432/mode/2up
    
    Runge was inspired by F. Stolze, "Die photographische Orstbestimmung 
    ohne Chronometer", which book he reviewed earlier in the some volume. 
    Unfortunately, my high school German is too rusty to get more than an 
    occasional glimmer of sense.
    http://www.archive.org/stream/zeitschriftfrve23vermgoog#page/n318/mode/2up
    
    Amazingly, the Stolze book also is online at archive.org. From the U.S. 
    Geodetic Survey document I infer Stolze explains how to use photography 
    to get latitude, local time, and lunar distance. Runge's experiment was 
    limited to lunar distance.
    http://www.archive.org/details/diephotographis00stolgoog
    
    Speaking of Runge, I believe he was none other than Carl Runge, of 
    Runge-Kutta fame. The latter did live in Hanover at the time and was a 
    noted spectroscopist, which would explain the spectrogram measuring machine.
    
    -- 
    I filter out messages with attachments or HTML.
    
    
    
    
    

       
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